The structure-function relationships of histidine decarboxylase from Lactobacillus 30a will be studied by x-ray diffraction techniques. This is an inducible bacterial amino acid decarboxylase found associated with the digestive tract of mammals. It produces large amounts of the neurotransmitter, histamine, although the physiological significance of this is not well understood. The enzyme possesses a number of unique properties, utilizing a covalently bound pyruvoyl moiety instead of pyridoxal-5'-phosphate. The pyruvoyl prosthetic group originates from an internal serine residue during an auto-activation process involving cleavage of a serine-serine peptide bond. We have crystals of wild-type histidine decarboxylase, its proenzyme, and of an active site analog of the native enzyme. We have recently computed a 3 angstroms resolution electron density map for the wild-type enzyme which has been successfully used to produce a chain tracing for this enzyme. This is the only amino acid decarboxylase or keto acid dependent enzyme for which x-ray structural data is available. Additional high resolution data will be collected for this system and the resulting model refined. The corresponding structural studies on the proenzyme form will be conducted and the two refined structures compared to study the unique mechanism of activation in these enzymes. Chemical modification of active site residues and structural analysis of active site analogs of the wild-type enzyme will be used in conjunction with amino acid sequence data on related systems to study the mechanism of action of these pyruvoyl dependent histidine decarboxylases. New crystallographic studies of pyridoxal-5'-phosphate dependent forms of histidine decarboxylase will also be undertaken during this grant period.